The present invention relates to an X-ray impinging position alignment method and an X-ray tomographic imaging method and apparatus, and more particularly to an X-ray impinging position alignment method for an X-ray emitting/detecting apparatus that emits an X-ray generated from an X-ray tube onto an X-ray detector through a collimator, and an X-ray tomographic imaging method and apparatus for performing imaging with the X-ray impinging position alignment.
In X-ray CT (computed tomography), an X-ray emitting/detecting apparatus for emitting an X-ray generated from an X-ray tube onto an X-ray detector through a collimator is rotated around (i.e., scans) a subject to be examined, and projection data for the subject is measured using the X-ray in a plurality of view directions around the subject to produce (i.e., reconstruct) a tomographic image based on the projection data.
The X-ray emitting apparatus emits an X-ray beam having a width in which an imaging range is contained and a certain thickness in the direction perpendicular to the width. The thickness of the X-ray beam is determined by the degree of opening of an X-ray passing aperture of the collimator.
The X-ray detecting apparatus detects the X-ray by a multi-channel X-ray detector comprising multiple X-ray detector elements arranged in an array in the direction of the X-ray beam width. The multi-channel X-ray detector has a length (i.e., width) corresponding to the X-ray beam width in the direction of the X-ray beam width, and a length (i.e., thickness) greater than the X-ray beam thickness in the direction of the X-ray beam thickness.
Some X-ray detectors comprise the X-ray detector element array having two rows to simultaneously obtain projection data for two slices. In such X-ray detectors, the two rows of the array are disposed adjacent to each other in parallel, and the X-ray beam impinges upon the detector equally apportioned in the thickness direction. Each thickness of the X-ray beam impinging upon each of the two rows of the array at the subjects isocenter determines the slice thickness of the tomographic image.
In the X-ray tube, an X-ray focus shifts due to thermal expansion caused by a temperature rise during use or the like, resulting in displacement of the X-ray beam in the thickness direction after passing through the collimator aperture. If the X-ray beam is displaced in the thickness direction, the distribution proportion of the X-ray beam thickness between the two rows of the array varies and the respective slice thicknesses for the subject projected on the two series of the array become unequal.
Thus, a technique is employed involving providing the two rows of the array with respective reference channels, monitoring the ratio between X-ray counts at the reference channels, detecting a shift in the X-ray impinging position if the ratio is not equal to one, and adjusting the collimator position, thereby controlling the X-ray impinging position to remain at a fixed position.
However, since the above technique for controlling the impinging position should be started only after the X-ray is emitted and a scan is started, the X-ray impinging position does not always coincide with a fixed position immediately after the beginning of the scan, or rather it may be shifted from the fixed position more often than not. Accordingly, there is a problem that an image initially obtained is subject to quality degradation.